Grayscale ?crashes? with too many features

Issue: The display ?crashes? mid refresh and it doesn't complete drawing the image. Lesser graphical activity will look ok but rapidly degrade, very light grayscale activity will stay as expected. Only occurs with grayscale mode, mono works fine (it seems - I don't have a way to confirm it isn't also having trouble somehow).
Arduino board: Adafruit Magtag but probably not relegated to just the Magtag - seems more e-ink than ucontroller related.
Arduino IDE version (found in Arduino -> About Arduino menu): 2.2.1, EPD library 4.5.3, rest of libraries the latest.
To trigger, compile the code in test.zip. It targets a display that's the Magtag size 296x128 (from memory, but prob near it). test.zip

Uncomment line 32 (which is one black box too far) and it'll show the screen mostly. Replace line 14 with 15 to see mono runs it with line 32 fine (but I need those sweet sweet 2 extra colors). Not sure what triggers it exactly, so the test case has all the things I do - text, lil bitmaps, and boxes of black. Too many boxes of black push it over the edge, but idk if it's their fault or the cumulative amount of "ink".

adafruit / Adafruit_EPD

Grayscale ?crashes? with too many features #80